What is RCU, Really?

The following sections provide some additional viewpoints and
information.

What is RCU?

RCU is a synchronization mechanism that was added to the Linux kernel
during the 2.5 development effort that is optimized for read-mostly
situations. Although RCU is actually quite simple once you understand it,
getting there can sometimes be a challenge. Part of the problem is that
most of the past descriptions of RCU have been written with the mistaken
assumption that there is "one true way" to describe RCU. Instead,
the experience has been that different people must take different paths
to arrive at an understanding of RCU. This document provides several
different paths, as follows:

People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
some point. People who prefer to start with an API that they can then
experiment with should focus on Section 2. People who prefer to start
with example uses should focus on Sections 3 and 4. People who need to
understand the RCU implementation should focus on Section 5, then dive
into the kernel source code. People who reason best by analogy should
focus on Section 6. Section 7 serves as an index to the docbook API
documentation, and Section 8 is the traditional answer key.

So, start with the section that makes the most sense to you and your
preferred method of learning. If you need to know everything about
everything, feel free to read the whole thing -- but if you are really
that type of person, you have perused the source code and will therefore
never need this document anyway. ;-)

The basic idea behind RCU is to split updates into "removal" and
"reclamation" phases. The removal phase removes references to data items
within a data structure (possibly by replacing them with references to
new versions of these data items), and can run concurrently with readers.
The reason that it is safe to run the removal phase concurrently with
readers is the semantics of modern CPUs guarantee that readers will see
either the old or the new version of the data structure rather than a
partially updated reference. The reclamation phase does the work of reclaiming
(e.g., freeing) the data items removed from the data structure during the
removal phase. Because reclaiming data items can disrupt any readers
concurrently referencing those data items, the reclamation phase must
not start until readers no longer hold references to those data items.

Splitting the update into removal and reclamation phases permits the
updater to perform the removal phase immediately, and to defer the
reclamation phase until all readers active during the removal phase have
completed, either by blocking until they finish or by registering a
callback that is invoked after they finish. Only readers that are active
during the removal phase need be considered, because any reader starting
after the removal phase will be unable to gain a reference to the removed
data items, and therefore cannot be disrupted by the reclamation phase.

So the typical RCU update sequence goes something like the following:

Remove pointers to a data structure, so that subsequent
readers cannot gain a reference to it.

Wait for all previous readers to complete their RCU read-side
critical sections.

At this point, there cannot be any readers who hold references
to the data structure, so it now may safely be reclaimed
(e.g., kfree()d).

Step (b) above is the key idea underlying RCU's deferred destruction.
The ability to wait until all readers are done allows RCU readers to
use much lighter-weight synchronization, in some cases, absolutely no
synchronization at all. In contrast, in more conventional lock-based
schemes, readers must use heavy-weight synchronization in order to
prevent an updater from deleting the data structure out from under them.
This is because lock-based updaters typically update data items in place,
and must therefore exclude readers. In contrast, RCU-based updaters
typically take advantage of the fact that writes to single aligned
pointers are atomic on modern CPUs, allowing atomic insertion, removal,
and replacement of data items in a linked structure without disrupting
readers. Concurrent RCU readers can then continue accessing the old
versions, and can dispense with the atomic operations, memory barriers,
and communications cache misses that are so expensive on present-day
SMP computer systems, even in absence of lock contention.

In the three-step procedure shown above, the updater is performing both
the removal and the reclamation step, but it is often helpful for an
entirely different thread to do the reclamation, as is in fact the case
in the Linux kernel's directory-entry cache (dcache). Even if the same
thread performs both the update step (step (a) above) and the reclamation
step (step (c) above), it is often helpful to think of them separately.
For example, RCU readers and updaters need not communicate at all,
but RCU provides implicit low-overhead communication between readers
and reclaimers, namely, in step (b) above.

So how the heck can a reclaimer tell when a reader is done, given
that readers are not doing any sort of synchronization operations???
Read on to learn about how RCU's API makes this easy.

There are many other members of the RCU API, but the rest can be
expressed in terms of these five, though most implementations instead
express synchronize_rcu() in terms of the call_rcu()
callback API.

The five core RCU APIs are described below, the other 18 will be enumerated
later. See the kernel docbook documentation for more info, or look directly
at the function header comments.

rcu_read_lock()

void rcu_read_lock(void);

Used by a reader to inform the reclaimer that the reader is
entering an RCU read-side critical section. It is illegal
to block while in an RCU read-side critical section, though
kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side
critical sections. Any RCU-protected data structure accessed
during an RCU read-side critical section is guaranteed to remain
unreclaimed for the full duration of that critical section.
Reference counts may be used in conjunction with RCU to maintain
longer-term references to data structures.

rcu_read_unlock()

void rcu_read_unlock(void);

Used by a reader to inform the reclaimer that the reader is
exiting an RCU read-side critical section. Note that RCU
read-side critical sections may be nested and/or overlapping.

synchronize_rcu()

void synchronize_rcu(void);

Marks the end of updater code and the beginning of reclaimer
code. It does this by blocking until all pre-existing RCU
read-side critical sections on all CPUs have completed.
Note that synchronize_rcu() will not
necessarily wait for
any subsequent RCU read-side critical sections to complete.
For example, consider the following sequence of events:

To reiterate, synchronize_rcu() waits only for ongoing RCU
read-side critical sections to complete, not necessarily for
any that begin after synchronize_rcu() is invoked.

Of course, synchronize_rcu() does not necessarily return
immediately after the last pre-existing RCU read-side critical
section completes. For one thing, there might well be scheduling
delays. For another thing, many RCU implementations process
requests in batches in order to improve efficiencies, which can
further delay synchronize_rcu().

Since synchronize_rcu() is the API that must figure out when
readers are done, its implementation is key to RCU. For RCU
to be useful in all but the most read-intensive situations,
synchronize_rcu()'s overhead must also be quite small.

The call_rcu() API is a callback form of
synchronize_rcu(),
and is described in more detail in a later section. Instead of
blocking, it registers a function and argument which are invoked
after all ongoing RCU read-side critical sections have completed.
This callback variant is particularly useful in situations where
it is illegal to block.

rcu_assign_pointer()

typeof(p) rcu_assign_pointer(p, typeof(p) v);

Yes, rcu_assign_pointer()is implemented as a macro, though it
would be cool to be able to declare a function in this manner.
(Compiler experts will no doubt disagree.)

The updater uses this function to assign a new value to an
RCU-protected pointer, in order to safely communicate the change
in value from the updater to the reader. This function returns
the new value, and also executes any memory-barrier instructions
required for a given CPU architecture.

Perhaps more important, it serves to document which pointers
are protected by RCU. That said, rcu_assign_pointer() is most
frequently used indirectly, via the _rcu list-manipulation
primitives such as list_add_rcu().

rcu_dereference()

typeof(p) rcu_dereference(p);

Like rcu_assign_pointer(), rcu_dereference()
must be implemented as a macro.

The reader uses rcu_dereference() to fetch an RCU-protected
pointer, which returns a value that may then be safely
dereferenced. Note that rcu_deference() does not actually
dereference the pointer, instead, it protects the pointer for
later dereferencing. It also executes any needed memory-barrier
instructions for a given CPU architecture. Currently, only Alpha
needs memory barriers within rcu_dereference()
-- on other CPUs,
it compiles to nothing, not even a compiler directive.

Common coding practice uses rcu_dereference() to copy an
RCU-protected pointer to a local variable, then dereferences
this local variable, for example as follows:

p = rcu_dereference(head.next);
return p->data;

However, in this case, one could just as easily combine these
into one statement:

return rcu_dereference(head.next)->data;

If you are going to be fetching multiple fields from the
RCU-protected structure, using the local variable is of
course preferred. Repeated rcu_dereference() calls look
ugly and incur unnecessary overhead on Alpha CPUs.

Note that the value returned by rcu_dereference() is valid
only within the enclosing RCU read-side critical section.
For example, the following is not legal:

Holding a reference from one RCU read-side critical section
to another is just as illegal as holding a reference from
one lock-based critical section to another! Similarly,
using a reference outside of the critical section in which
it was acquired is just as illegal as doing so with normal
locking.

As with rcu_assign_pointer(), an important function of
rcu_dereference() is to document which pointers are protected
by RCU. And, again like rcu_assign_pointer(),
rcu_dereference()
is typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu().

Relation Among RCU Primitives

The following diagram shows how each API communicates among the
reader, updater, and reclaimer.

The RCU infrastructure observes the time sequence of rcu_read_lock(),
rcu_read_unlock(), synchronize_kernel(),
and call_rcu() invocations in
order to determine when (1) synchronize_kernel()
invocations may return
to their callers and (2) call_rcu()
callbacks may be invoked. Efficient
implementations of the RCU infrastructure make heavy use of batching in
order to amortize their overhead over many uses of the corresponding APIs.

There are no fewer than three RCU mechanisms in the Linux kernel; the
diagram above shows the first one, which is by far the most commonly used.
The rcu_dereference() and rcu_assign_pointer()
primitives are used for
all four mechanisms, but different defer and protect primitives are
used as follows:

This section shows a simple use of the core RCU API to protect a
global pointer to a dynamically allocated structure. More typical
uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.

struct foo {
int a;
char b;
long c;
};
DEFINE_SPINLOCK(foo_mutex);
struct foo *gbl_foo;
/*
* Create a new struct foo that is the same as the one currently
* pointed to by gbl_foo, except that field "a" is replaced
* with "new_a". Points gbl_foo to the new structure, and
* frees up the old structure after a grace period.
*
* Uses rcu_assign_pointer() to ensure that concurrent readers
* see the initialized version of the new structure.
*
* Uses synchronize_rcu() to ensure that any readers that might
* have references to the old structure complete before freeing
* the old structure.
*/
void foo_update_a(int new_a)
{
struct foo *new_fp;
struct foo *old_fp;
new_fp = kmalloc(sizeof(*fp), GFP_KERNEL);
spin_lock(&foo_mutex);
old_fp = gbl_foo;
*new_fp = *old_fp;
new_fp->a = new_a;
rcu_assign_pointer(gbl_foo, new_fp);
spin_unlock(&foo_mutex);
synchronize_rcu();
kfree(old_fp);
}
/*
* Return the value of field "a" of the current gbl_foo
* structure. Use rcu_read_lock() and rcu_read_unlock()
* to ensure that the structure does not get deleted out
* from under us, and use rcu_dereference() to ensure that
* we see the initialized version of the structure (important
* for DEC Alpha and for people reading the code).
*/
int foo_get_a(void)
{
int retval;
rcu_read_lock();
retval = rcu_dereference(gbl_foo)->a;
rcu_read_unlock();
return retval;
}

So, to sum up:

Use rcu_read_lock() and rcu_read_unlock()
to guard RCU read-side critical sections.

Within an RCU read-side critical section, use
rcu_dereference() to dereference RCU-protected pointers.

Use some solid scheme (such as locks or semaphores) to
keep concurrent updates from interfering with each other.

Use rcu_assign_pointer() to update an RCU-protected pointer.
This primitive protects concurrent readers from the updater,
not concurrent updates from each other! You therefore still
need to use locking (or something similar) to keep concurrent
rcu_assign_pointer() primitives from interfering
with each other.

Use synchronize_rcu()after
removing a data element from an
RCU-protected data structure, but before reclaiming/freeing
the data element, in order to wait for the completion of all
RCU read-side critical sections that might be referencing that
data item.

See checklist.txt in the Documentation/RCU
directory of the Linux kernel
source tree for additional rules to follow when using RCU.

In the example above, foo_update_a()
blocks until a grace period elapses.
This is quite simple, but in some cases one cannot afford to wait so
long -- there might be other high-priority work to be done.

In such cases, one uses call_rcu() rather than
synchronize_rcu().
The call_rcu() API is as follows:

This function invokes func(head) after a grace period has elapsed.
This invocation might happen from either softirq or process context,
so the function is not permitted to block. The foo struct needs to
have an rcu_head structure added, perhaps as follows:

struct foo {
int a;
char b;
long c;
struct rcu_head rcu;
};

The foo_update_a() function might then be written as follows:

/*
* Create a new struct foo that is the same as the one currently
* pointed to by gbl_foo, except that field "a" is replaced
* with "new_a". Points gbl_foo to the new structure, and
* frees up the old structure after a grace period.
*
* Uses rcu_assign_pointer() to ensure that concurrent readers
* see the initialized version of the new structure.
*
* Uses call_rcu() to ensure that any readers that might have
* references to the old structure complete before freeing the
* old structure.
*/
void foo_update_a(int new_a)
{
struct foo *new_fp;
struct foo *old_fp;
new_fp = kmalloc(sizeof(*fp), GFP_KERNEL);
spin_lock(&foo_mutex);
old_fp = gbl_foo;
*new_fp = *old_fp;
new_fp->a = new_a;
rcu_assign_pointer(gbl_foo, new_fp);
spin_unlock(&foo_mutex);
call_rcu(&old_fp->rcu, foo_reclaim);
}

The container_of() primitive is a macro that, given a pointer into a
struct, the type of the struct, and the pointed-to field within the
struct, returns a pointer to the beginning of the struct.

The use of call_rcu() permits the caller of foo_update_a() to
immediately regain control, without needing to worry further about the
old version of the newly updated element. It also clearly shows the
RCU distinction between updater, namely foo_update_a(), and reclaimer,
namely foo_reclaim().

The summary of advice is the same as for the previous section, except
that we are now using call_rcu() rather than
synchronize_rcu():

Use call_rcu()after removing a data element from an
RCU-protected data structure in order to register a callback
function that will be invoked after the completion of all RCU
read-side critical sections that might be referencing that
data item.

Again, see checklist.txt for additional rules governing the
use of RCU.

One of the nice things about RCU is that it has extremely simple "toy"
implementations that are a good first step towards understanding the
production-quality implementations in the Linux kernel. This section
presents two such "toy" implementations of RCU, one that is implemented
in terms of familiar locking primitives, and another that more closely
resembles "classic" RCU. Both are way too simple for real-world use,
lacking both functionality and performance. However, they are useful
in getting a feel for how RCU works. See kernel/rcupdate.c for a
production-quality implementation, and see:

http://www.rdrop.com/users/paulmck/RCU

for papers describing the Linux kernel RCU implementation. The OLS'01
and OLS'02 papers are a good introduction, and the dissertation provides
more details on the current implementation.

5A. "TOY" IMPLEMENTATION #1: LOCKING

This section presents a "toy" RCU implementation that is based on
familiar locking primitives. Its overhead makes it a non-starter for
real-life use, as does its lack of scalability. It is also unsuitable
for realtime use, since it allows scheduling latency to "bleed" from
one read-side critical section to another.

However, it is probably the easiest implementation to relate to, so is
a good starting point.

The rcu_read_lock() and rcu_read_unlock()
primitive read-acquire
and release a global reader-writer lock. The synchronize_rcu()
primitive write-acquires this same lock, then immediately releases
it. This means that once synchronize_rcu() exits, all RCU read-side
critical sections that were in progress before synchonize_rcu() was
called are guaranteed to have completed -- there is no way that
synchronize_rcu() would have been able to write-acquire the lock
otherwise.

It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired. Note also that rcu_read_lock() is immune
from deadlock (an important property of RCU). The reason for this is
that the only thing that can block rcu_read_lock()
is a synchronize_rcu().
But synchronize_rcu() does not acquire any locks while
holding rcu_gp_mutex,
so there can be no deadlock cycle.

Quick Quiz #1:

Why is this argument naive? How could a deadlock
occur when using this algorithm in a real-world Linux
kernel? How could this deadlock be avoided?

5B. "TOY" EXAMPLE #2: CLASSIC RCU

This section presents a "toy" RCU implementation that is based on
"classic RCU". It is also short on performance (but only for updates) and
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
kernels. The definitions of rcu_dereference()
and rcu_assign_pointer()
are the same as those shown in the preceding section, so they are omitted.

Note that rcu_read_lock() and rcu_read_unlock()
do absolutely nothing.
This is the great strength of classic RCU in a non-preemptive kernel:
read-side overhead is precisely zero, at least on non-Alpha CPUs.
And there is absolutely no way that rcu_read_lock() can possibly
participate in a deadlock cycle!

The implementation of synchronize_rcu()
simply schedules itself on each
CPU in turn. The run_on()
primitive can be implemented straightforwardly
in terms of the sched_setaffinity()
primitive. Of course, a somewhat less
"toy" implementation would restore the affinity upon completion rather
than just leaving all tasks running on the last CPU, but when I said
"toy", I meant toy!

So how the heck is this supposed to work???

Remember that it is illegal to block while in an RCU read-side critical
section. Therefore, if a given CPU executes a context switch, we know
that it must have completed all preceding RCU read-side critical sections.
Once all CPUs have executed a context switch, then all preceding
RCU read-side critical sections will have completed.

So, suppose that we remove a data item from its structure and then invoke
synchronize_rcu(). Once synchronize_rcu()
returns, we are guaranteed
that there are no RCU read-side critical sections holding a reference
to that data item, so we can safely reclaim it.

Quick Quiz #2:

Give an example where Classic RCU's read-side
overhead is negative.

Quick Quiz #3:

If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
PREEMPT_RT, where normal spinlocks can block???

Although RCU can be used in many different ways, a very common use of
RCU is analogous to reader-writer locking. The following unified
diff shows how closely related RCU and reader-writer locking can be.

Either way, the differences are quite small. Read-side locking moves
to rcu_read_lock() and rcu_read_unlock(),
update-side locking moves from
from a reader-writer lock to a simple spinlock, and a synchronize_rcu()
precedes the kfree().

However, there is one potential catch: the read-side and update-side
critical sections can now run concurrently. In many cases, this will
not be a problem, but it is necessary to check carefully regardless.
For example, if multiple independent list updates must be seen as
a single atomic update, converting to RCU will require special care.

Also, the presence of synchronize_rcu() means that the RCU version of
delete() can now block. If this is a problem,
there is a callback-based
mechanism that never blocks, namely call_rcu(), that can be used in
place of synchronize_rcu().

The RCU APIs are documented in docbook-format header comments in the
Linux-kernel source code, but it helps to have a full list of the
APIs, since there does not appear to be a way to categorize them
in docbook. Here is the list, by category.

Markers for RCU read-side critical sections:

rcu_read_lock
rcu_read_unlock
rcu_read_lock_bh
rcu_read_unlock_bh

RCU pointer/list traversal:

rcu_dereference
list_for_each_rcu (to be deprecated in favor of
list_for_each_entry_rcu)
list_for_each_safe_rcu (deprecated, not used)
list_for_each_entry_rcu
list_for_each_continue_rcu (to be deprecated in favor of new
list_for_each_entry_continue_rcu)
hlist_for_each_rcu (to be deprecated in favor of
hlist_for_each_entry_rcu)
hlist_for_each_entry_rcu

The acronym "RCU" stands for "read-copy update", which in turn derives
from the way that RCU is used to update a linked structure in place.
A thread wishing to do this uses the following steps:

create a new structure,

copy the data from the old structure into the new one, and save
a pointer to the old structure,

modify the new, copied, structure

update the global pointer to refer to the new structure,

sleep until the operating system kernel determines that there
are no readers left using the old structure, for example,
in the Linux kernel, by using synchronize_rcu(), and finally

upon awakening, deallocate the old structure.

So the structure is read concurrently with a thread
copying in order to do an update, hence the name
"read-copy update". The abbreviation "RCU" was one of many contributions
by the Linux community. Other names for similar techniques include
passive serialization and MP defer by VM/XA programmers and generations
by K42 and Tornado programmers.

CPU 1 is interrupted, and the irq handler
attempts to acquire problematic_lock.

The system is now deadlocked.

One way to avoid this deadlock is to use an approach like
that of CONFIG_PREEMPT_RT, where all normal spinlocks
become blocking locks, and all irq handlers execute in
the context of special tasks. In this case, in step 4
above, the irq handler would block, allowing CPU 1 to
release rcu_gp_mutex, avoiding the deadlock.

Even in the absence of deadlock, this RCU implementation
allows latency to "bleed" from readers to other
readers through synchronize_rcu(). To see this,
consider task A in an RCU read-side critical section
(thus read-holding rcu_gp_mutex), task B blocked
attempting to write-acquire rcu_gp_mutex, and
task C blocked in rcu_read_lock() attempting to
read_acquire rcu_gp_mutex. Task A's RCU read-side
latency is holding up task C, albeit indirectly via
task B.

Imagine a single-CPU system with a non-CONFIG_PREEMPT
kernel where a routing table is used by process-context
code, but can be updated by irq-context code (for example,
by an "ICMP REDIRECT" packet). The usual way of handling
this would be to have the process-context code disable
interrupts while searching the routing table. Use of
RCU allows such interrupt-disabling to be dispensed with.
Thus, without RCU, you pay the cost of disabling interrupts,
and with RCU you don't.

One can argue that the overhead of RCU in this
case is negative with respect to the single-CPU
interrupt-disabling approach. Others might argue that
the overhead of RCU is merely zero, and that replacing
the positive overhead of the interrupt-disabling scheme
with the zero-overhead RCU scheme does not constitute
negative overhead.

In real life, of course, things are more complex. But
even the theoretical possibility of negative overhead for
a synchronization primitive is a bit unexpected. ;-)

Quick Quiz #3:

If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
PREEMPT_RT, where normal spinlocks can block???

Answer:

Just as PREEMPT_RT permits preemption of spinlock
critical sections, it permits preemption of RCU
read-side critical sections. It also permits
spinlocks blocking while in RCU read-side critical
sections.

Why the apparent inconsistency? Because it is it
possible to use priority boosting to keep the RCU
grace periods short if need be (for example, if running
short of memory). In contrast, if blocking waiting
for (say) network reception, there is no way to know
what should be boosted. Especially given that the
process we need to boost might well be a human being
who just went out for a pizza or something. And although
a computer-operated cattle prod might arouse serious
interest, it might also provoke serious objections.
Besides, how does the computer know what pizza parlor
the human being went to???

ACKNOWLEDGEMENTS

My thanks to the people who helped make this human-readable, including
Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood.